This invention pertains generally to optical switches, and in particular to an optical switch that uses a Fourier transform lens to direct light from an array of optical inputs to an array of optical outputs.
Optical fibers are used in a variety of systems, including telecommunication networks and phased array antennas. For such applications, large optical interconnect switches having one thousand or more optical inputs and outputs are needed. In these switches, each one of the inputs should be capable of being connected to any one of the outputs, so that the outputs represent a desired permutation of the inputs.
A number of optical switches have been proposed. In U.S. Pat. No. 4,512,036, Laor describes an optical switch whose fiber optic inputs are attached to piezo-electric benders that can aim the input fibers at different outputs. However, the response time for this switch can be slow, and it can be difficult to reproducibly move the fibers to the desired positions.
In U.S. Pat. No. 5,440,654, Lambert describes a fiber optic switching system having transparent beam deflectors for directing each input beam to the desired output. The beam deflectors are made from an electro-optical phase shifting medium, such as a liquid crystal. To deflect a beam, a diffraction grating is created in the electro-optical material that redirects the beam. This switch is inconvenient because the input beams must be monochromatic, and the color of light being switched must be known in advance.
Levinson, in U.S. Pat. No. 4,580,973, describes an optical matrix switch having m optical to inputs arranged in a one-dimensional array, and a one-dimensional array of n optical outputs placed orthogonal to the light paths of the inputs. An mxc3x97n matrix of electromechanically actuated mirrors is positioned so that each input beam can be controllably deflected to any of the outputs. This switch grows prohibitively complex as the number of inputs and outputs is increased, since the number of mirrors needed is equal to the product nm.
In U.S. Pat. No. 4,365,863, Broussaud discloses a switch having a two-dimensional array of inputs facing a two-dimensional array of outputs. Each input beam is controllably deflected toward one of the outputs.
All of the above switches suffer from the limitation that they cannot be reduced in size because of diffraction limits. To miniaturize one of these switches, the radius of the light beams transmitted through the switch must be reduced. However, a beam of width w and wavelength xcex will inevitably spread with a diffraction angle xcex8 given approximately by: sin xcex8=xcex/w. Therefore, as one attempts to make the switch smaller, the optical beams diverge, causing much of the light to be lost rather than transmitted to the outputs as required.
Another class of switches uses an acousto-optic medium to direct light beams from the optical inputs to the outputs. Such switches are disclosed in Weverka, U.S. Pat. No. 5,165,104, and in Harris, xe2x80x9cMultichannel Acousto-Optic Crossbar Switch,xe2x80x9d Applied Optics 30 (1991) 4245-4256. The acousto-optic effect used in these switches, however, requires that the optical inputs form a linear array, and that the outputs form another linear array perpendicular to the array of inputs. Therefore the acousto-optic switches are large and cumbersome when many inputs and outputs are used.
It is therefore a primary object of the present invention to provide a compact switch that overcomes the diffraction limitations of the prior art. An optical switch comprises a two-dimensional array of optical inputs, a Fourier transform lens, and a two-dimensional array of optical outputs. Each of the optical inputs emits an optical beam that is transmitted through the Fourier transform lens to one of the optical outputs. A first deflection means gives each of the optical beams a respective direction of incidence upon the Fourier transform lens. The optical output to which a given beam travels depends on the beam""s respective direction of incidence. A second deflection means deflects the optical beams after they have been transmitted through the Fourier transform lens and renders the beams parallel, thereby enabling the beams to couple efficiently to the optical outputs.
In the present switch, the optical inputs are coupled one-to-one with the optical outputs. By controlling the first deflection means, any directions of incidence may be given to the optical beams; therefore, the optical outputs correspond to any desired permutation of the optical inputs.
In the preferred embodiment, the switch further comprises a first polarizing beam splitter for splitting each of the optical beams into two beam components. In this embodiment, the first deflection means comprises two deflector arrays, one for each of the two beam components. After striking the deflector arrays, the two beam components are recombined by the first polarizing beam splitter, and are subsequently transmitted through the Fourier transform lens. The preferred embodiment also comprises a second polarizing beam splitter located on the transmission side of the Fourier transform lens.
The second polarizing beam splitter splits and recombines the beams, and the second deflection means comprises two more deflector arrays.
Each of the deflection means preferably comprises at least one Micro-Electro-Mechanical Systems, or MEMS, deflector array. The optical inputs and outputs preferably comprise optical fibers. The MEMS arrays and optical fibers allow the switch to be made extremely compact. The Fourier transform lens overcomes diffraction limitations, since the lens causes the spreading optical beams to reconverge.